I used embroidery stabilizer that the lady at JoAnn's cut me (1/4 of a yard). The name brand is stitch-n-tear and it is the kind that you just put behind the fabric and secure with the hoop.

Jennie Hart is selling stabilizer through her Sublime Stitching website now too and it might be better. I thought that this stabilizer was a little rough. You really need stabilizer for t-shirt material though.